Alumina is a raw material used in the production of many different types of ceramic materials, catalyst supports and refractories. Most commercial alumina is produced according to the "Bayer Process", a well known process for the production of alumina from bauxite. In the Bayer Process, bauxite is mixed with hot concentrated NaOH which dissolves some of the alumina, silica and other components of the bauxite. Most of the silica and other materials are reprecipitated and eliminated as a solid waste referred to as "red mud". The dissolved alumina is then separated in solution from remaining solids and crystalized as alumina trihydrate, alpha-Al.sub.2 O.sub.3.3H.sub.2 O ("gibbsite"). Because it is formed in a sodium hydroxide environment, the gibbsite contains a significant amount (usually 0.3 to 0.4%) soda, Na.sub.2 O. (All percentages herein are by weight unless otherwise noted.) In addition, the economics of the Bayer Process are such that significant amounts of other impurities such as silica are tolerated in the gibbsite product. A typical analysis of gibbsite from the Bayer Process is shown in Table I. In accordance with the usual practice, impurities are expressed as the stable oxide form.
TABLE I ______________________________________ Impurities Concentration, % ______________________________________ Na.sub.2 O 0.3 CaO 0.01 ZnO 0.004 SiO.sub.2 0.04 Fe.sub.2 O.sub.3 0.01 ______________________________________
A number of other minor oxides are also present, in quantities of less than a few hundred ppm each. When the Bayer Process alumina trihydrate is calcined to produced anhydrous alumina, Al.sub.2 O.sub.3, the impurities are concentrated by a factor of about 1.5.
While most of the commercial hydrated alumina is produced by the Bayer Process as described, it is possible to produce hydrated alumina by other methods. To the extent that such other methods result in the inclusion of unacceptably high levels of one or more impurities in the hydrated alumina, the purification process of the present invention will be applicable to purification of those materials. For brevity herein, however, the process of this invention will be described in terms of the purification of Bayer Process-produced gibbsite, although it is to be understood the it is acceptable to all impure hydrated aluminas.
For most alumina uses, such as electrolytic production of aluminum metal or formation of ordinary ceramic products and refractories, the gibbsite is entirely adequate even with these high levels of impurities present. For a number of applications, however, these impurity levels (particularly the high soda level) are unacceptable. These applications include products intended for use in the electronics industry, as supports for certain types of catalytic materials, as synthetic sapphire and as translucent bodies. Depending upon the particular application or product, maximum alumina impurity levels for materials such as soda, silica or iron oxide may be as low as 0.002%.
At present most commercially available aluminas of low soda (Na.sub.2 O) content are derived from the gibbsite made by the Bayer Process. Reduction in soda levels is accomplished by one of several methods that can attain, at best, minimum Na.sub.2 O levels of 0.02-0.05%. The most common type of method involves a partial conversion of gibbsite to an active form of alumina which is treated with an aqueous solution of a chloride source such as ammonium choride, weak hydrochloric acid or aluminum chloride. After removal of the liquid phase the resulting solid containing adsorbed chlorides is calcined at temperatures greater than 1000.degree. C. to yield alumina with soda contents of 0.02-0.9%. This type of process is described, for instance, in German Offen. No. 2,816,1914; German Offen. No. 1,276,017; and Canadian Pat. No. 795,706. These methods are restricted to production of alumina suitable for applications which can tolerate more than 0.02% Na.sub.2 O.
A different method of soda reduction involves calcination of gibbsite in the presence of a substance containing at least 10% silica at 1260.degree. C. The soda is selectively concentrated in the silica which is then separated by screening from the alumina. The product alumina has a soda level of 0.02%, and also has an elevated quantity of SiO.sub.2 of about 0.02%. See U.S. Pat. No. 3,106,452.
A process for recovering high purity alumina from waste streams from the hydrochloric acid etch of aluminum for electronics uses has been reported by Ruthner, et al in the published proceedings of the ICSOBA Third International Congress (Nice, 19173), pages 546-555. Very low levels of impurities are attained, but for economic reasons the method is applicable only when a highly pure waste stream of aqueous aluminum chloride is available. A number of other methods have been proposed that start with pure aluminum metal, organoaluminum compounds or alums. These in general start with a high cost material or generate products not recycleable to the process when calcined and are therefore not applicable to commercial production.